KR100530944B1 - Separation of c2f6 from cf4 by adsorption on activated carbon - Google Patents

Abstract

The present invention relates to an improved process for removing C ₂ F ₆ as impurities from CF ₄ produced by the reaction of a CF ₄ containing gas, preferably F ₂ with carbon. The improved method

Contacting the CF ₄ containing gas containing the C ₂ F ₆ impurities with activated carbon having a CCl ₄ activity coefficient of 43 to 55 in the adsorbent layer to perform adsorption of the C ₂ F ₆ impurities; And

Recovering the purified CF ₄ product in the effluent from the adsorbent bed

It includes.

Description

Separation of C2F6 from CF4 by adsorption on activated carbon {SEPARATION OF C2F6 FROM CF4 BY ADSORPTION ON ACTIVATED CARBON}

Historically, high purity CF ₄ , which is used to etch silica in the manufacture of integrated circuits, has been induced by direct fluorination of carbon. Purification of the formed CF ₄ gas, including the removal of impurities such as C ₂ F ₆ , is carried out by temperature swing adsorption using a zeolite layer such as NaX (13X). CF ₄ is co-sorbed with C ₂ F ₆ on 13X zeolite. When the zeolite layer is saturated with C ₂ F ₆ , the N ₂ purge passes through the spent adsorbent bed to recover a portion of co-adsorbed CF ₄ from the adsorbent bed. CF ₄ , which is less strongly adsorbed than C ₂ F ₆ , is first desorbed into the N ₂ purge gas and then C ₂ F ₆ is desorbed. N ₂ purge gas containing desorbed CF ₄ and a small amount of desorbed C ₂ F ₆ was subjected to 13X zeolite until the C ₂ F ₆ concentration in this effluent rose to an unacceptable level in the CF ₄ / N ₂ mixture. Will pass through the second layer of. Through this process, about half of the co-adsorbed CF ₄ can be recovered before the C ₂ F ₆ concentration in the effluent becomes too high.

Representative patents for the separation of fluorocarbon gas include the following.

US 6,187,077 discloses a process for separating at least one of CF ₄ and C ₂ F ₆ from a gas containing SF ₆ and at least one of NF ₃ , CHF ₃ and N ₂ . The process comprises (1) passing a feedstock stream containing various impurities through a glassy membrane to produce a retentate stream rich in SF ₆ and at least one of CF ₄ and C ₂ F ₆ , and (2) SF ₆ Adsorbing to contact the retentate stream with an adsorbent effective to produce a product stream rich in at least one of CF ₄ and C ₂ F ₆ . Representative adsorbents include zeolites, preferably X-type zeolites, activated carbon such as BPL (data sheet exhibits CCl ₄ activity of 60-65), PCB (data sheet exhibits CCl ₄ activity of 60), BAC, F-300, F-400, BPL, RB2 (data sheet exhibits CCl ₄ activity of 65), of which PCB is particularly preferred. The patent also discloses polymer adsorbent resins and carbon molecular sieves.

US Pat. No. 5,523,499 discloses a process for the purification of C ₂ F ₆ contaminated with CClF ₃ and CHF ₃ impurities by adsorption. C ₂ F ₆ gas contaminated with impurities is brought into contact with an adsorbent comprising zeolite molecular sieve and activated carbon. Preferred activated carbons such as BPL from Calgon Corporation and Type UU from Barneby and Sutcliffe Corp. have particle sizes of 4 to 325 mesh.

Japanese Patent Application No. 54-62867 (Publication 55-154925) discloses a process for purifying CF ₄ containing CF ₃ Cl as an impurity. The process includes laser irradiation of the gas stream and absorption of photons in the fluorine compound to convert CFCl ₃ to C ₂ F ₆ and Cl ₂ . C ₂ F ₆ is then removed via distillation or adsorption.

There is a need in the industry for adsorbents that allow long onstream time for a given column size, and an adsorbent with higher selectivity for other fluorocarbon impurities other than CF ₄ products. This improved adsorbent improves CF ₄ recovery.

The present invention relates to an improved process for removing C ₂ F ₆ as impurities from CF ₄ containing gases, preferably CF ₄ produced by the reaction between F ₂ and carbon. This improved method

Contacting the CF ₄ containing gas with activated carbon having a CCl ₄ activity factor of 43 to 55 in an adsorbent layer to effect selective adsorption of the C ₂ F ₆ impurities; And

Recovering the purified CF ₄ product in the effluent from the adsorbent bed

It includes.

Significant advantages of the method include

That impurities can be removed from a CF ₄ containing gas stream contaminated with fluorocarbon impurities,

Irreversible adsorption of CF ₄ allows the selective adsorption of contaminants C ₂ F ₆ without substantial loss,

Can provide a long operating time in the adsorption process, and

Effective removal of fluorocarbon impurities from CF ₄ products can be achieved

This includes.

Carbon tetrafluoride is produced by direct fluorination of carbon. The process produces a reaction product that contains many impurities and must be removed. Various impurities in the reaction product include unreacted fluorine, HF, SF ₆ and carbon fluorides (eg CHF ₃ and C ₂ F ₆ ). Impurity C ₂ F ₆ is produced due to incomplete reaction of carbon with F ₂ . Typical CF ₄ feed gas concentrations gatneunde the C ₂ F ₆ of 500 ppm to 5000 ppm in the CF _4, a C ₂ F ₆ is to be reduced to less than 0.5 ppm for commercial high purity CF _4.

The process of removing contaminants C ₂ F ₆ from the CF ₄ gas stream to produce commercial high purity CF ₄ is achieved by contacting the gas stream with activated carbon having a CCl ₄ activity factor of 43-55. If the binding force of the activated carbon is below the CCl ₄ activity factor of 43, then some of the C ₂ F ₆ may pass through the adhesive layer to contaminate the CF ₄ product. If the CCl ₄ activity coefficient is 55 or more, the binding power of activated carbon is so high that the CF ₄ product may be irreversibly retained in the adsorbent layer of activated carbon. Such activity levels are characterized by a C ₂ F ₆ adsorption capacity of 0.15 mmole C ₂ F ₆ / g activated carbon (mmole C ₂ F ₆ / g), preferably at least 0.02 mmole C ₂ F at a temperature of 25-30 ° C. ₆ / g, most preferably at least 0.025 mmol C ₂ F ₆ / g.

Carbon tetrachloride activity of the adsorbent is a standard measurement (ASTM test method D3467-99) that measures the gravimetric saturation capacity of the adsorbent. Carbon tetrachloride (CCl ₄ ) activity is referred to herein as the ratio of the weight of the sample to the weight of CCl ₄ adsorbed by the activated carbon sample (%) when the activated carbon is saturated with CCl ₄ under the conditions listed in such ASTM test method. Defined and also incorporated by reference in this specification. Recently, the use of CCl ₄ activity has been replaced by the characterization of butane adsorption on adsorbents. Butane activity is equivalent to CCl ₄ activity and is expressed by the following equation for CCl ₄ activity.

Butane Activity = CCl ₄ Activity / 2.52

Thus, the corresponding CCl ₄ activity can be obtained by multiplying the butane butane activity of the adsorbent by 2.52.

Activated carbon suitable for use in the removal of impurity C ₂ F ₆ typically has a particle size range of 0.5 to 3 mm in diameter.

The inlet temperature of the feedstock entering the adsorption bed is in the range of 0-100 ° C, and the inlet pressure of the feedstock entering the adsorption bed is in the range of 1-20 atm absolute pressure. Preferred operating temperatures range from 20 to 50 ° C., and preferred operating pressures range from 2 to 10 atm.

The regeneration process of the spent activated carbon layer for reuse can be carried out according to generally recognized procedures. Desorption can be achieved at pressures of 0.1 to 2 atm absolute and temperatures of 50 to 300 ° C. Thus, this process is suitable for both thermal swing and pressure swing adsorption / desorption processes. The regeneration process of the spent activated carbon layer using thermal swing adsorption allows the use of non-reactive gases as a purge. Such gases include N ₂ , Ar, He, H ₂ and mixtures thereof.

43. The adsorbent having a carbon tetrachloride adsorption capacity of up to 55 gatgin preferential selectivity with respect to C ₂ F ₆ than CF _4, however, occurs when CF ₄ being part of the suction is lost. Proper recovery of CF ₄ adsorbed by activated carbon in the adsorbent layer can be achieved by passing purge gas through the spent activated carbon layer. CF ₄ desorbs into the purge gas in preference to C ₂ F ₆ . To remove residual C ₂ F ₆ from the purge gas, the purge gas from the spent gas is passed through a new layer of activated carbon, where the residual C ₂ F ₆ is adsorbed. Improved recovery of CF ₄ is achieved. By using a plurality of layers using a pressure homogenization step, the recovery can be improved.

The following examples are provided to illustrate preferred embodiments of the invention and are not intended to limit the scope of the invention.

Example 1

CF on activated carbon ₄ Selective adsorption of

To determine the effectiveness of various activated carbons to remove C ₂ F ₆ from CF ₄ , 198 g of Pacific Activated Carbon (CCl ₄ activity factor of 45) is 3 feet or about 1 meter in length. Was charged into a 1 "diameter phosphorus. Feedstock CF ₄ gas was passed through the column, and the effluent from the column was subjected to gas chromatography (GC) to CF ₄ concentration (%) and C ₂ F ₆ concentration (ppm). Was analyzed.

Initially, the column packed with Pacific Activated Carbon (CCl ₄ activity coefficient of 45) was saturated with CF ₄ and the temperature was kept constant. A gas mixture containing 1000 ppm C ₂ F ₆ and 300 ppm SF _{6 in} CF _{4 was} then passed through a column operating at 100 sscm at atmospheric pressure and 25-30 ° C. Effluent from the column was analyzed every 3 minutes until there was a breakthrough. Breakthrough time was defined as the time at which C ₂ F ₆ was detected at 3 ppm in the effluent by GC. After 20.0 hours of operation, a C ₂ F ₆ breakthrough was observed. This is consistent with a C ₂ F ₆ capacity of 0.026 mmole C ₂ F ₆ / g of Pacific activated carbon at breakthrough.

Example 2

CF from Pacific Activated Carbon ₄ Selective removal of

Immediately after the C ₂ F ₆ breakthrough in Example 1, the feedstock gas was switched to an N ₂ flow of 100 sscm to desorb the Pacific activated carbon layer. The column was operated at ambient temperature during desorption. During the initial desorption of CF ₄ from Pacific Activated Carbon, the level of impurities in the purge gas consistently retained C ₂ F ₆ at 40 ppm. After purging for 106 minutes, the concentration of CF _{4 in} N ₂ dropped below 10%, at which time desorption was stopped.

Desorbed CF ₄ in N ₂ can be recovered by conventional means, such as by passing the CF ₄ / N ₂ mixture through a second activated carbon column to remove C ₂ F ₆ and to separate it from the purge N _{2 at} cryogenic temperatures. Could.

Example 3

CF with 13X Zeolite ₄ Selective adsorption of

The procedure of Example 1 was repeated except that 278 g of 13X (CCl ₄ activity coefficient was 38) was used as the adsorbent instead of Pacific Activated Carbon. A CF ₄ feedstock gas contaminated with both SF ₆ and C ₂ F ₆ was used and breakthrough was observed after 13.0 hours. This is consistent with a C ₂ F ₆ dose of 0.012 mmole C ₂ F ₆ / g.

Example 4

CF from 13X zeolite ₄  collection

A CF ₄ recovery experiment similar to Example 2 was performed on the column used in Example 3. Nitrogen was passed through the zeolite layer and after 60 minutes, the concentration of CF _{4 in} N ₂ dropped to 10% or less. Desorption was stopped at this point. A concentration of C ₂ F _{6 in} nitrogen was observed at 60 ppm, gradually increasing to 285 ppm at the end of the desorption step for CF ₄ recovery.

Example 5

CF on Barneby-Sutcliffe Type 205A Activated Carbon ₄ Adsorption of

The procedure of Example 1 was repeated except that different activated carbons were used. Breakthrough experiments were performed under the same conditions using a Barneby-Sutcliffe type 205A activated carbon adsorbent with a CCl ₄ activity coefficient of 53. The C ₂ F ₆ dose measured from the breakthrough curve was 0.16 mmole C ₂ F ₆ / g. The active density of type 205A activated carbon is basically the same as that of Pacific activated carbon (0.5 g / cc).

Example 6

C ₂ F ₆  Capacity vs CCl ₄  Curve about activity

Plots for CCl ₄ activity of adsorption mmole C ₂ H ₆ / g activated carbon versus activated carbon are plotted based on the results of the above examples and are illustrated in FIG. 1.

The results in the plot show one surprising result. The C ₂ F ₆ capacity of the adsorbent falls within the preferred range with an activity coefficient of at least 43 to 55, with an activity coefficient of 44 to 50, with an optimal CCl ₄ activity coefficient of about 45. This is unexpected because those skilled in the art expected that the larger the saturation capacity of the adsorbent, the larger the C ₂ F ₆ capacity would be. As can be seen from the data, activated carbon with high C ₂ F ₆ capacity does not have equally high CF ₄ capacity. The activity level of 0.015 mmole C ₂ F ₆ / g activated carbon, preferably from about 0.02 to about 0.03 mmole C ₂ F ₆ / g activated carbon, allows CF ₄ to be contained in the effluent and pass through, and C ₂ F ₆ The purge treatment from activated charcoal without substantial contamination due to permits significant levels of recovery.

In summary of the above embodiment, the Pacific aekti Bay suited carbon having a CCl ₄ activity factor of 45 is the performance surpasses than the Barneby-Sutcliffe activated carbon both with zeolite 13X and 53 of CCl ₄ activity coefficient having a CCl ₄ activity factor of 38 It is clear that. However, Barneby-Sutcliffe activated carbon has a coefficient of activity sufficient to remove C ₂ F ₆ in some applications. Comparing Example 1 with Example 3, the active activated carbon has been shown to have a C ₂ F ₆ capacity of at least twice as compared to 13X based on mmole C ₂ F ₆ / g. In addition, due to the packing density difference, the Pacific Activated Carbon also allows longer operating times of> 50% to remove C ₂ F ₆ from CF ₄ than 13X. Moreover, these Pacific activated carbons adsorb the residual impurity SF ₆ even more strongly than 13X.

Another feature of using activated carbon having an activity coefficient of 43 to 55 is significant when comparing Example 2 to Example 4. These examples show that CF ₄ recovery at ambient temperature using N ₂ purge can be achieved. In the case of using activated carbon having a CCl ₄ activity coefficient of 43 to 55 as compared with 13X, desorbed C ₂ F ₆ is not desorbed in the purge gas, and SF ₆ is substantially desorbed.

The examples also show that the presence of low levels of SF _{6 in} the feedstock does not adversely affect the C ₂ F ₆ doses measured in Example 1 or 5.

Although the invention has been described with reference to a number of preferred embodiments, the entire scope of the invention should fall within the scope of the appended claims.

The present invention provides an improved process for removing C ₂ F ₆ as impurities from CF ₄ containing gases, preferably CF ₄ produced by the reaction between F ₂ and carbon, thereby contaminating CF contaminated with fluorocarbon impurities. ₄ contains may remove impurities from the gas stream, the CF ₄ ratio can selectively adsorb the contaminant C ₂ F ₆ without substantial loss by irreversible adsorption, it is possible to provide a long operating time in the adsorption step, and CF ₄ Effective removal of fluorocarbon impurities from the product can be achieved.

1 is a graph of C ₂ F ₆ adsorption on activated carbon versus CCl ₄ activity of such activated carbon adsorbents.

Claims (10)

A method for removing C ₂ F ₆ as impurities from a CF ₄ containing gas, wherein the CF ₄ containing gas is contacted with an adsorbent in an adsorbent bed for selectively adsorbing C ₂ F ₆ , wherein CF ₄ is introduced into the effluent from the adsorbent bed. In the method of recovering as a contained product, Contacting the CF ₄ containing gas containing the C ₂ F ₆ impurities with an adsorbent composed of activated carbon having a CCl ₄ activity coefficient of 43 to 55 in the adsorbent layer to perform adsorption of the C ₂ F ₆ impurities; And Recovering the purified CF ₄ product in the effluent from the adsorbent bed Method comprising a. The method of claim 1, wherein the CF ₄ containing gas has C ₂ F ₆ of about 500 ppm to 5000 ppm as an impurity. The method of claim 2, wherein the activated carbon has a particle size of 0.5 to 3 mm in diameter. The method of claim 3 wherein the adsorbent bed is operated at an inlet temperature of 0-100 ° C. and an inlet pressure of 1-20 atm absolute pressure of the feedstock entering the adsorbent bed. The method of claim 2 wherein enhanced recovery of CF ₄ is passed through the purge gas to the adsorbent bed to the desorption previous C ₂ F ₆ which is achieved by selective desorption with the CF ₄ from the adsorbent layer method. The method of claim 5 wherein said purge gas is nitrogen. The method of claim 6, wherein the CF ₄ retained in the adsorbent layer is desorbed at a pressure of 0.1 to 2 atmospheric pressure and a temperature of 50 to 300 ° C. 8. A method for removing C ₂ F ₆ as impurities from a CF ₄ containing gas, wherein the CF ₄ containing gas is contacted with an adsorbent in an adsorbent bed for selectively adsorbing C ₂ F ₆ , wherein CF ₄ is introduced into the effluent from the adsorbent bed. In the method of recovering as a contained product, The C ₂ F ₆ or higher CF ₄ 0.015-containing gas in the adsorbent layer to a temperature 25 ~ 30 ℃ mmole containing the impurity C ₂ F ₆ / g is brought into contact with activated carbon having a C ₂ F ₆ capacity of the active carbon wherein the C ₂ F ₆ Carrying out adsorption of impurities; And Recovering the purified CF ₄ product in the effluent from the adsorbent bed Method comprising a. The method of claim 8, wherein the CF ₄ containing gas has C ₂ F ₆ of about 500 ppm to 5000 ppm as impurities. The method of claim 8, wherein the activated carbon has a C ₂ F ₆ capacity of at least 0.02-0.03 mmole C ₂ F ₆ / g activated carbon.